Wind shear accidents have become an increasingly recognized problem in aviation. The accidents are commonly associated with a descent to an airport wherein the aircraft leaves a mass of air moving with a given velocity and direction and descends into wind conditions nearer the ground which are radically different both in velocity and direction. The unexpected increase or decrease in speed causes the aircraft to overshoot or undershoot the runway. On take off the wind shear can, in some cases, be so severe as to lower the air speed to the point where the aircraft stalls or settles back to the ground.
Classical aviation flight theory has, for the most part, taught that such accidents are impossible on the grounds that airplanes fly only with respect to the air mass surrounding them. Accordingly, it was argued, if the wind changes, the aircraft moves with it and no change in flight attitude is perceived. This theory worked well in the past because airplanes were relatively light so that they did indeed move easily with the wind. Lighter airplanes also tend to have fairly powerful engines relative to their inertial mass and therefore can accelerate and compensate for any changes quickly. Thus, given the typical magnitude of wind shear conditions encountered in nature this problem has been largely ignored.
With the development of larger, faster, and heavier aircraft the problem has been growing steadily worse. The faster an airplane travels the faster it passes through changing wind conditions. These greater wind shears demand a greater ability on the part of the aircraft to respond to the new wind. However, newer aircraft are heavier and aerodynamically cleaner so that, to the contrary, they respond less quickly to wind changes. The worsening problem is exemplified by a recent crash at the Boston airport.
On Apr. 7. 1975 a heavy DC-10 aircraft was making an approach to Boston airport and attempting to follow the fixed path through space determined by the glide slope. Wind shears of up to 9.1 knots per hundred feet were encountered during the descent causing the aircraft to encounter an ever increasing headwind. Attempting to stay on the guide slope the autopilot continually reduced the power settings to compensate for the lift of the added headwind. Just about the time that the aircraft became stabilized on the glide slope with the new wind it became necessary to immediately increase the power settings to maintain the proper rate of descent. The autopilot probably would have increased the power sufficiently but unfortunately at this moment the pilots disengaged the autopilot and took over the flight visually which is a routine procedure at this point in an approach. Before the pilots could appreciate the significance of the aircraft's low power condition it was too late and they crashed short of the runway. What the pilots did not know, and could not know because it was masked by the autopilot reducing the throttle settings, was how fast the air speed was changing in comparison to how slowly the inertial speed was changing. It is the purpose of my invention to provide a system that will warn of this condition.